A widely employed type of tube bending machine embodies an elongated machine bed at one end of which is mounted apparatus for achieving draw bending or press bending of the tube. A multiple bend tube commonly has a number of bends located at different points along the tube and having the planes of different bends angularly shifted about the tube axis. Other variables in the bending process, whether performed by a draw or press bending, include the degree of bend and the radius of the bend. The latter two variables commonly are handled by the bending head. The position of the bend along the axis of the tube and the angular position of the plane of bend are often handled by a mechanism which grasps the tube and advances it toward the bending head to position the point of bend at the proper location with respect to dies in the bending head. The tube grasping mechanism also rotates the tube relative to the bending head to attain a selected plane of bend. Such tube handling mechanism must be simple, reliable and accurate. It must be lightweight for a fast response time, particularly where a number of bends are to be made in rapid succession by a machine that is entirely automatically controlled. Thus, many bending machines in the past have embodied separate driving mechanisms for carriage travel and chuck rotation. Heavy, expensive, screw-type drives have been employed, and these have been provided in duplicate for the driving of the chuck and the driving of the carriage. In some arrangements, one or more heavy driving motors are carried by the carriage.
Precision ball screw drives for a machine having a typical ten foot length of travel are massive and expensive. Where motors are mounted on the carriage, not only is the motor cost increased by use of plural motors, but the carriage and all supporting and driving structures must be stronger, heavier, more expensive and more difficult to precisely and rapidly control. In some prior arrangements, one motor has been employed for two drives, but these have still required duplicating of the driving connection between the motor and the several driven members.
A simple, lighter weight tension drive, such as a belt or chain, has not been employed partly because suitable drives from a chain mechanism have not been available. Another problem with the chain or belt drive is the looseness or compliance of the mechanical coupling of such an apparatus, particularly in view of the required length of carriage travel. For example, with a machine having a carriage capable of traveling ten feet, an endless chain having a length of approximately twenty feet is required to transmit the driving force from a fixed motor to the carriage. Because of the large number of joints in such a chain, the play or looseness inherent in each such joint, and the weight of the workpiece to be moved, the actual position of the driven element, the carriage or the chuck, will lag the commanded position of the driving motor shaft. Such lagging error has its greatest adverse affect during deceleration.
Although the looseness or compliance of the relatively long mechanical coupling between the driving motor and the driven carriage or chuck is more pronounced in a flexible endless loop chain drive, it also occurs in any long mechanical driving connection mechanism. A gear drive also exhibits some degree of looseness or play between successive interengaging gears of a gear chain, and an elongated shaft of a screw drive will exhibit a comparable torsional compliance as the shaft winds and unwinds about its own axis.
The longer the mechanical driving connection, the greater the looseness of the connection. A long chain drive for example, has so much play in its many interconnected joints, that when the commanded motor velocity is caused to decrease, the velocity of the driven member at the other end of the long mechanical drive, will not decrease immediately. That is, actual deceleration of the driven member lags deceleration of the remote driving motor. Such a lag will increase with increase of momentum (either mass or velocity) of the driven parts, mass of the interposed mechanical driving element (e.g. a long screw shaft, a set of interconnected gears, or a long chain), and the compliance, play, slack, or extensibility of the driving connection. Partly because of such lagging of velocity changes, adequate precision of position control has not been achieved without massive and expensive driving structures.
A belt or chain drive is preferred for a number of reasons, including the desired use of small, lightweight parts. Nevertheless, control mechanisms have not enabled such drives to exhibit satisfactory positioning of the driven member.
Conventional servo systems embody automatically reversing drive motors that enable oscillation or hunting of the servo driven system as moving parts approach the desired position, under control of an error signal. The moving parts tend to overshoot, throwing the system into reverse in which condition it may again overshoot the desired position and continue to hunt back and forth for a period of time. Various methods of minimizing such hunting have been devised, including systems involving viscous damping and error rate damping. In viscous damping, a restraining force is applied to the moving parts, but this introduces a constant error in steady state condition. In an error rate damping system, the damping force is derived from the rate of change of the error of voltage and is subject to changes in frequency of the controlling current, among other disadvantages. In any event, these damping systems are concerned with a directly driven element and apply the damping control directly to such driven element. Such damped servo systems cannot effectively control an element that is driven by a relatively loose or compliant driving connection from the servo control damped drive motor. Further, a more expensive automatically reversing motor is required.
Where braking systems have been applied for damping, they have been connected to operate directly on the motor shaft. Various controlled braking forces have been employed, including those applying braking only below certain speeds or in certain directions of motion, but none have been adequate for accurate position control.
It has been suggested that positioning may be achieved by applying a constant braking force at a certain distance from the final position, but the many variables that govern stopping distance, including variation of a manually fixed braking torque, and mass and speed of the moving parts, prevent positioning with satisfactory repeatability and precision.
Accordingly, it is an object of the present invention to afford position control of increased precision without the disadvantages of previous arrangements.